Abstract

A field-size effect of physical doses was studied in scanning irradiation with carbon ions. For the target volumes of , , and , the doses along the beam axis within the spread-out Bragg peaks reduced to 99.4%, 98.2%, and 96.0% of the dose for the target of , respectively. The present study revealed that the observed reductions can be compensated for by adopting the three-Gaussian form of lateral dose distributions for the pencil beam model used in the treatment planning system. The parameters describing the form were determined through the irradiation experiments making flat concentric squared frames with a scanned carbon beam. Since utilizing the three-Gaussian model in the doseoptimization loop is at present time consuming, the correction for the field-size effect should be implemented as a “predicted-dose scaling factor.” The validity of this correction method was confirmed through the irradiation of a target of .

Received 26 January 2009Revised 09 April 2009Accepted 29 April 2009Published online 09 June 2009

Acknowledgments:

The authors are very grateful to the HIMAC operation staff for their skillful work. They would like to thank the members of the Medical Physics Research Group for their fruitful advice on the research. The authors are also grateful to the reviewers and editors for their comments and advice. This work was carried out as a part of the Research Project with Heavy Ions at HIMAC.

Article outline:I. INTRODUCTIONII. MATERIALS AND METHODSII.A. Pencil beam modelII.A.1. Single-Gaussian form of the beamII.A.2. Multi-Gaussian form of the beamII.B. Experimental setupII.B.1. Field-size dependence of the doses within the SOBP regionII.B.2. Determination of coefficients describing the Gaussian beam profilesII.C. Estimation of errors propagated to and III. RESULTS AND DISCUSSIONIII.A. Field-size dependence of the dose with single-Gaussian modelIII.B. Determination of coefficients describing each Gaussian formIII.C. Errors propagated to and III.D. Recalculation of the doses with three-Gaussian beam modelIII.E. Test of two-Gaussian beam modelIII.F. Doseoptimization with three-Gaussian beam modelIII.G. Correction of field-size effect with predicted-dose scaling factorIV. CONCLUSIONS